Very early on, computer software came to be categorized as “operating system” software or “application” software. Broadly speaking, an application is software meant to perform a specific task for the computer user such as solving a mathematical equation or supporting word processing. The operating system is the software that manages and controls the computer hardware. The goal of the operating system is to make the computer resources available to the application programmer while at the same time, hiding the complexity necessary to actually control the hardware.
The operating system makes the resources available via functions that are collectively known as the Application Program Interface or API. The term API is also used in reference to a single one of these functions. The functions are often grouped in terms of what resource or service they provide to the application programmer. Application software requests resources by calling individual API  functions. API functions also serve as the means by which messages and information provided by the operating system are relayed back to the application software.
In addition to changes in hardware, another factor driving the evolution of operating system software has been the desire to simplify and speed application software development. Application software development can be a daunting task, sometimes requiring years of developer time to create a sophisticated program with millions of lines of code. For a popular operating system such as Microsoft Windows®, application software developers write thousands of different applications each year that utilize the operating system. A coherent and usable operating system base is required to support so many diverse application developers.
Often, development of application software can be made simpler by making the operating system more complex. That is, if a function may be useful to several different application programs, it may be better to write it once for inclusion in the operating system, than requiring dozens of software developers to write it dozens of times for inclusion in dozens of different applications. In this manner, if the operating system supports a wide range of common functionality required by a number of applications, significant savings in applications software development costs and time can be achieved.
Regardless of where the line between operating system and application software is drawn, it is clear that for a useful operating system, the API between the operating system and the computer hardware and application software is as important as efficient internal operation of the operating system itself. 
Over the past few years, the universal adoption of the Internet, and networking technology in general, has changed the landscape for computer software developers. Traditionally, software developers focused on single-site software applications for standalone desktop computers, or LAN-based computers that were connected to a limited number of other computers via a local area network (LAN). Such software applications were typically referred to as “shrink wrapped” products because the software was marketed and sold in a shrink-wrapped package. The applications utilized well-defined APIs to access the underlying operating system of the computer.
As the Internet evolved and gained widespread acceptance, the industry began to recognize the power of hosting applications at various sites on the World Wide Web (or simply the “Web”). In the networked world, clients from anywhere could submit requests to server-based applications hosted at diverse locations and receive responses back in fractions of a second. These Web applications, however, were typically developed using the same operating system platform that was originally developed for standalone computing machines or locally networked computers. Unfortunately, in some instances, these applications do not adequately transfer to the distributed computing regime. The underlying platform was simply not constructed with the idea of supporting limitless numbers of interconnected computers.
To accommodate the shift to the distributed computing environment being ushered in by the Internet, Microsoft Corporation is developing a network software platform known as the “.NET” platform (read as “Dot Net”). The platform allows developers to create Web services that will execute over the  Internet. Such a dynamic shift requires a new ground-up design of an entirely new API.
In response to this challenge, the inventors developed a unique set of API functions for Microsoft's .NET™ platform.